In the middle of taking an XUD to bits and found this:
...the other 3 are perfect and look like new. This was the cylinder where I had a glow plug snap off in the head. I wonder if some debris has fallen down into the cylinder

The engine did have a small amount of blow by, I had put that down to the crappy fibre head gasket it had fitted, but perhaps this was the reason. The middle ring on the piston was slightly stuck as well.
The piston crown shows signs of damage:
